parasite

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Biology An organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host.
2. n. One who habitually takes advantage of the generosity of others without making any useful return.
3. n. One who lives off and flatters the rich; a sycophant.
4. n. A professional dinner guest, especially in ancient Greece.

Century Dictionary and Cyclopedia

1. n. Originally, one who frequents the tables of the rich and earns his welcome by flattery; hence a hanger-on; a fawning fiatterer; a sycophant.
2. n. Specifically In zoö., an animal that lives in or on and at the expense of another animal called technically the host; also, by extension, an animal which lives on or with, but not at the expense of, its host: in the latter sense, more precisely designated inquilince or commensal (see these words). , Particularly, an insect which lives either upon or within another insect during its earlier stages, eating and usually destroying its host. In botany, a plant which grows upon another plant or upon an animal, and feeds upon its juices. See parasitic, and cut under Cercospora. There is scarcely any animal that may not or does not serve as the host of parasites, and some parasites are themselves the hosts of other parasites. (See hyperparasite.) Parasites form no technical group of animals, since representatives of almost any class or order, from protozoans to vertebrates, may be parasitic. Most of the leading divisions of animals, however, include some members, whether genera, families, orders, or even classes, whose habit is extensively or exclusively parasitic. Thus, among protozoans, the Gregarinida are parasites. Among worms, many families, some orders, or even classes, are entirely parasitic, furnishing the most formidable and frequent parasites of man and domestic animals. Very many of the lower crustaceans are parasites, especially upon fishes, mollusks, etc., and upon one another; while some of the highest crustaceans are modified parasites, or commensals, as the little crabs that live in oyster-shells. Among arachnidans, the whole class or order of acarids or mites is essentially parasitic, though including many forms which lead an independent life. Insects furnish many of the parasites, especially of terrestrial animals, as vertebrates, and some are parasites of other insects. One order of insects, the Anoplura or lice, is thoroughly parasitic, and other orders furnish parasitic families or genera. Insects and crustaceans both belong to the phylum Arthropoda, and it may be said that as a rule insects furnish the arthropod parasites of land-animals, and crustaceans those of water-animals, or terrestrial and aquatic “lice” respectively. Few mollusks are parasitic, but Entoconcha mirabilis, a gastropod found in holothurians, is an example. Very few vertebrates are parasites, but hags (Myxine) bore into fishes, fishes of the genus Fierasfer crawl into the intestines of holothurians, and some other fishes exhibit a kind of parasitism. Parasites not constituting any natural division of animals, it follows that, as such, they are not naturally divisible into zoölogical groups. They are, however, conveniently called enloparasites or ectoparasites, according as they live in or on their hosts, or Entozoa and Epizoa, upon the same grounds. According to the extent or degree of their parasitism, they are also known as parasites proper and commensals or inquilines (see above). Among the most remarkable parasites are the males of some species which have their own females as hosts, as among cirripeds. Such males are known as complemental males, one or more of which are carried about by the female in her vulva, they being of insignificant size and to all intents and purposes mere male parts of her. The above-mentioned parasites are exclusive of all those many animals which are parasitic upon plants, as gall-insects and the like; and also of those birds which are parasitic to the extent of laying their eggs in other birds’ nests, requiring their progeny to be hatched and brought up by foster-parents, as cuckoos and cowbirds. See cuts under Cecrops, Entoniscus, Epizoa, Platypsylla, and Stylops
3. n. In teratology See autosite.

Wiktionary

1. n. biology A (generally undesirable) living organism that exists by stealing the resources produced/collected by another living organism.
2. n. pejorative A person who relies on other people's efforts and gives little back (originally a sycophant).

GNU Webster's 1913

1. n. One who frequents the tables of the rich, or who lives at another's expense, and earns his welcome by flattery; a hanger-on; a toady; a sycophant.
2. n. A plant obtaining nourishment immediately from other plants to which it attaches itself, and whose juices it absorbs; -- sometimes, but erroneously, called epiphyte.
3. n. A plant living on or within an animal, and supported at its expense, as many species of fungi of the genus Torrubia.
4. n. An animal which lives during the whole or part of its existence on or in the body of some other animal, feeding upon its food, blood, or tissues, as lice, tapeworms, etc.
5. n. An animal which steals the food of another, as the parasitic jager.
6. n. An animal which habitually uses the nest of another, as the cowbird and the European cuckoo.

WordNet 3.0

1. n. an animal or plant that lives in or on a host (another animal or plant); it obtains nourishment from the host without benefiting or killing the host
2. n. a follower who hangs around a host (without benefit to the host) in hope of gain or advantage